Fill level measurement devices, and in particular propagation time sensors for fill level measurement which function according to the radar, TDR (time domain reflectometry) or ultrasound principle generally have an energy consumption that fluctuates significantly and is caused by the alternation between energy-intensive measurement phases and evaluation phases or dead times having a considerably lower energy requirement.
If such a fill level measurement device is connected to a two-wire loop, the energy intake resulting from the two-wire operation (4-20 mA or a constant current having superposed digital communication) is at least temporarily constant and the resultant electrical power currently available can be less than the power currently required in the measurement phase.
This requires a buffer store for storing energy in order to bridge a temporary power deficit.
Propagation time sensors that comprise an accumulator for supplying energy for a long time are also known, which accumulator, however, is not charged via a two-wire loop. Said accumulator has to be charged separately at a given time.